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From: ersys!markt@nro.cs.athabascau.ca (Mark Tarrabain)
Newsgroups: comp.lang.c
Subject: Re: Portability vs. Endianness
Message-ID: <Nogsy1w164w@ersys.uucp>
Date: 14 Mar 91 07:44:22 GMT
References: <1991Mar12.105451.19488@dit.upm.es>
Organization: Edmonton Remote Systems, Edmonton, AB, Canada
Lines: 55

esink@turia.dit.upm.es writes:

> Given the following :
> 
> long var;
> unsigned char Bytes[4];
> 
> 
> Is there a portable way to move the value held in var
> into the memory space pointed to by Bytes, with the restriction
> that the representation be in Most Significant Byte first
> format ?  I am well aware that sizeof(long) may not be 4.  I
> want the value contained in var converted to a 68000
> long word, and I want the code fragment to run on any
> machine.  The solution must be ANSI C.
> 

try the following code fragment: (no guarantees mind you, It's late at
night and I'm kinda wired on caffeine)

union swap_type
  {
  long lvalue;
  char cvalue[4];
  };
int hifirst()
  {
  union swap_type dummy;
  dummy.lvalue = 1;
  return dummy.cvalue[3];
  }
void put_var_to_bytes(long var,char Bytes[4])
  {
  union swap_type dummy;
  dummy.lvalue = var;
  if (!hifirst())
    {
    char temp;
    temp = dummy.cvalue[0];
    dummy.cvalue[0] = dummy.cvalue[3];
    dummy.cvalue[3] = temp;
    temp = dummy.cvalue[1];
    dummy.cvalue[1] = dummy.cvalue[2];
    dummy.cvalue[2] = temp;
    }
  Bytes[0] = dummy.cvalue[0];
  Bytes[1] = dummy.cvalue[1];
  Bytes[2] = dummy.cvalue[2];
  Bytes[3] = dummy.cvalue[3];
  }

this is slightly machine dependant - it assumes that the size of a long
is the same as 4 characters. (which should be the case on a 68000 
anyways)
        >> Mark